Minecraft is an award winning building game and something of an educational phenomenon. It's an addictive game where players build a 3D landscape by adding and destroying blocks. Play this free java version to get your class creating and imagining. No registration required. Play the multi-player version at http://www.minecraft.net/classic/list, but a free registration is needed to play as a team.
http://ictmagic.wikispaces.com/ICT+%26+Web+Tools